DDMWORKS-01 Lower Duncan River Argenta Slough Erosion Protection – Ecological Inventory of Argenta Slough

Author: N. Wright, L. Ballin, A. Newbury, D. Lacroix

The regulatory agencies (Fisheries and Oceans Canada and Ministry of Forests, Lands and Natural Resources) do not support proceeding with any proposed physical works to armour the eroding bend in Lower Duncan River. Regulators agreed to continue erosion monitoring and the creation of an ecological inventory of the areas of the slough that are threatened by a breach. The Ecological Inventory was presented to regulators in May 2017. One of the outcomes of the presentation was for BC Hydro to recommend options in lieu of physical works. BC Hydro reviewed the options with the regulators in the fall of 2018 and agreed to provide a contribution to the acquisition of conservation property in lieu of physical works. The TOR resubmission was approved January 2019 and implementation is in progress.

The Argenta Slough Ecological Inventory is summarized below and in Table 11. There are a number of data gaps and limitations that restrict wetland delineation and classification, and impede an overall quantitative assessment of ecological function and condition of the Lower Argenta Slough; these have been outlined throughout the report and summarized below.

Wetland Delineation and Classification
The Lower Argenta Slough is a wetland complex that supports shallow water, seasonally flooded marsh, and flood/riparian forest ecosystems. Wetland ecosystem types and vegetation communities were identified and delineated to the extent possible in consideration of minimal background information, including limited data on vegetation species and soil composition. Ecosystems were also delineated on photographs from 2009 and 2011. Further, the biogeoclimatic classifications and site series in the ICHdw1 were in the process of being reclassified with more detailed inventory information at the time the mapping for this project was conducted. Lastly, it is expected that the vegetation composition of the Lower Argenta Slough is changing and that delineations may not best represent the current state of the slough.

Wetland Function and Condition

Hydrology and Connectivity
Due to the lack of existing water level and flow data, assessment of wetland hydrological function and condition was largely qualitative. Inferences on the hydrologic condition of the Lower Argenta Slough were made from locally available climate and hydrology data and reports.
Local climate data show warmer air temperatures in both winter and the growing season in recent years, coupled with increased precipitation for all months except April and July. An increase in air temperature has likely resulted in an increase in evapotranspiration rates, potentially resulting in greater water level drawdown during the growing season. These changes have likely resulted in alterations to the vegetation community composition.
Water levels within the slough are highly affected by Kootenay Lake backwatering during the spring freshet; however, the duration and timing of these floods is variable. After the spring freshet, water levels in the slough are expected to slowly recede through late summer to perennial low levels in winter. Lower Argenta Slough may receive groundwater inputs from Lower Duncan River when river stage is high.
From a hydrological perspective, wetland and riparian habitat within Lower Argenta Slough is highly connected during periods of high water level, particularly during the spring freshet when the slough is flooded by Kootenay Lake, and less connected during low water periods. However, the timing and duration of this connectivity has not been documented.
Temperature data is currently being collected at the Lower Slough. Changes to the temperature regime could be due to a number of reasons, including groundwater inputs, lowering of water levels due to evaporation or drainage, and/or river flooding. In the past, beaver activity has had the effect of ponding water in Upper Argenta Slough and reducing water levels in Lower Argenta Slough (Herbison 2006); however, the effect of beaver activity on the hydrology of the site has not been documented in recent years.

Vegetation Composition
Ecosystems at-risk occurring in the Lower Argenta Slough include blue-listed Common cattail marsh (Wm05) and Great bulrush communities (Wm06), and likely include the red-listed black cottonwood Spruce Red-osier dogwood (Fm02) or black cottonwood/ common snowberry roses (Fm01) and Swamp horsetail Beaked sedge (Wm51) communities and may include the three-way sedge community (Wm51). It is currently unknown if any rare plants occur in Lower Argenta Slough. Considering the expected soil nutrient and moisture regime, it is possible that other communities at-risk, as well as rare plants occur in the Lower Argenta Slough.

Wildlife Use
Three at-risk species have been confirmed to breed in Argenta Slough: American Bittern, Western Toad and Painted Turtle; however no American Bittern or Painted Turtle nests sites have been detected downstream of Beaver Dam 1. Lower Argenta Slough has been confirmed to provide rearing habitat to Western Toads. An additional three at-risk species have been detected in Argenta Slough and potentially breed there: Eared Grebe, Horned Grebe and American Water Shrew. Furthermore, Northern Rubber Boas have been detected near Argenta Slough and potentially breed in the vicinity.
Argenta Slough provides foraging habitat to a number of at-risk species, including: Tundra Swan, Great Blue Heron, Lewiss Woodpecker, Olive-sided Flycatcher and Bobolink. Argenta Slough also provides migratory stopover and avian overwintering habitat. Argenta Slough falls within a legally designated UWR; however, Lower Argenta Slough is unlikely to provide critical winter foraging habitat for Mule Deer due to a lack of snow interception cover and exposed winter vegetation. All of these at-risk, regionally significant, and migratory species may have a high to moderate dependency on Lower Argenta Slough; however, targeted surveys have not been conducted and thus, there is a lack of understanding of critical wildlife habitat functions provided by Lower Argenta Slough.

The Argenta Slough Ecological Inventory is summarized below and in Table 11. There are a number of data gaps and limitations that restrict wetland delineation and classification, and impede an overall quantitative assessment of ecological function and condition of the Lower Argenta Slough; these have been outlined throughout the report and summarized below.

Wetland Delineation and Classification
The Lower Argenta Slough is a wetland complex that supports shallow water, seasonally flooded marsh, and flood/riparian forest ecosystems. Wetland ecosystem types and vegetation communities were identified and delineated to the extent possible in consideration of minimal background information, including limited data on vegetation species and soil composition. Ecosystems were also delineated on photographs from 2009 and 2011. Further, the biogeoclimatic classifications and site series in the ICHdw1 were in the process of being reclassified with more detailed inventory information at the time the mapping for this project was conducted. Lastly, it is expected that the vegetation composition of the Lower Argenta Slough is changing and that delineations may not best represent the current state of the slough.

Wetland Function and Condition

Hydrology and Connectivity
Due to the lack of existing water level and flow data, assessment of wetland hydrological function and condition was largely qualitative. Inferences on the hydrologic condition of the Lower Argenta Slough were made from locally available climate and hydrology data and reports.
Local climate data show warmer air temperatures in both winter and the growing season in recent years, coupled with increased precipitation for all months except April and July. An increase in air temperature has likely resulted in an increase in evapotranspiration rates, potentially resulting in greater water level drawdown during the growing season. These changes have likely resulted in alterations to the vegetation community composition.
Water levels within the slough are highly affected by Kootenay Lake backwatering during the spring freshet; however, the duration and timing of these floods is variable. After the spring freshet, water levels in the slough are expected to slowly recede through late summer to perennial low levels in winter. Lower Argenta Slough may receive groundwater inputs from Lower Duncan River when river stage is high.
From a hydrological perspective, wetland and riparian habitat within Lower Argenta Slough is highly connected during periods of high water level, particularly during the spring freshet when the slough is flooded by Kootenay Lake, and less connected during low water periods. However, the timing and duration of this connectivity has not been documented.
Temperature data is currently being collected at the Lower Slough. Changes to the temperature regime could be due to a number of reasons, including groundwater inputs, lowering of water levels due to evaporation or drainage, and/or river flooding. In the past, beaver activity has had the effect of ponding water in Upper Argenta Slough and reducing water levels in Lower Argenta Slough (Herbison 2006); however, the effect of beaver activity on the hydrology of the site has not been documented in recent years.

Vegetation Composition
Ecosystems at-risk occurring in the Lower Argenta Slough include blue-listed Common cattail marsh (Wm05) and Great bulrush communities (Wm06), and likely include the red-listed black cottonwood Spruce Red-osier dogwood (Fm02) or black cottonwood/ common snowberry roses (Fm01) and Swamp horsetail Beaked sedge (Wm51) communities and may include the three-way sedge community (Wm51). It is currently unknown if any rare plants occur in Lower Argenta Slough. Considering the expected soil nutrient and moisture regime, it is possible that other communities at-risk, as well as rare plants occur in the Lower Argenta Slough.

Wildlife Use
Three at-risk species have been confirmed to breed in Argenta Slough: American Bittern, Western Toad and Painted Turtle; however no American Bittern or Painted Turtle nests sites have been detected downstream of Beaver Dam 1. Lower Argenta Slough has been confirmed to provide rearing habitat to Western Toads. An additional three at-risk species have been detected in Argenta Slough and potentially breed there: Eared Grebe, Horned Grebe and American Water Shrew. Furthermore, Northern Rubber Boas have been detected near Argenta Slough and potentially breed in the vicinity.
Argenta Slough provides foraging habitat to a number of at-risk species, including: Tundra Swan, Great Blue Heron, Lewiss Woodpecker, Olive-sided Flycatcher and Bobolink. Argenta Slough also provides migratory stopover and avian overwintering habitat. Argenta Slough falls within a legally designated UWR; however, Lower Argenta Slough is unlikely to provide critical winter foraging habitat for Mule Deer due to a lack of snow interception cover and exposed winter vegetation. All of these at-risk, regionally significant, and migratory species may have a high to moderate dependency on Lower Argenta Slough; however, targeted surveys have not been conducted and thus, there is a lack of understanding of critical wildlife habitat functions provided by Lower Argenta Slough.